DNA extracted from high-passage (H) herpes simplex virus (HSV) contains unique endonuclease restriction (endo R) fragments resistant to Hind III cleavage. DNA extracted from DNase-treated virions has a greater density in cesium chloride and contains some Vero cell DNA sequences. Differences are detected in endo R fragments in syncytial (syn) variants of HSV. These differences vary according to the host cell the syn variants are derived from. Specific endo R Eco R1 fragments of HSV type 1 will transform mouse 3T3 cells. The transformed cells have been shown to be positive for HSV information.